1. Field of the Invention
The present invention relates to a pump unit for transferring a fluid with a micropump, and a breathing assistance device using the pump unit.
2. Description of the Related Art
A breathing assistance device such as a respirator is being used in the field of medical care. The breathing assistance device employs systems including a controlled ventilation system used for a patient who cannot breathe spontaneously (a patient under general anesthesia or cardiopulmonary resuscitation, or a patient in a critical condition), an assisted ventilation system for generating a positive pressure in a respiratory passage in response to the spontaneous breathing of a patient, an assist-control ventilation system using the assisted ventilation and the controlled ventilation in combination, and a high-frequency oscillation ventilation for realizing a very small amount of single ventilation of from 1 to 2 ml/kg by causing a gas to be supplied to a respiratory passage to oscillate at a frequency of from 5 to 40 Hz.
This breathing assistance device is also used for a patient suffering from a sleep respiratory disorder. This respiratory disorder is caused by blockage of a respiratory passage as a result of relaxation of the muscle of the respiratory passage and a resultant lowered position of root part of a tongue or a soft palate. Applying positive pressure to a respiratory passage also relieves the symptom of a patient suffering from the respiratory disorder of this type.
A breathing assistance device of any type requires a pump unit to generate a positive pressure in a respiratory passage. A blower for transferring a gas by rotating a fan, a cylinder pump for transferring a gas by making reciprocating motion of a piston or the like is used as a power source for the pump unit.
Meanwhile, the pump unit used in a conventional breathing assistance device is of a relatively large a size. Accordingly, the breathing assistance device is housed in a box-shaped casing, and is placed beside a user when it is used. This makes it difficult to realize the compact size of the breathing assistance device.
A pump unit used in a breathing assistance device makes the following control as shown in FIG. 18, for example. The pump unit increases pressure (generates positive pressure) rapidly at a high flow rate in an initial stage during inspiratory operation, and thereafter, maintains a constant flow rate while assisting in the inspiration while further increasing the pressure. Further, during expiratory operation, the pump unit reduces pressure (generates negative pressure) rapidly at a high flow rate, and reduces a flow rate gradually if the pressure turns to decrease to avoid a burden on a lung. This control is only an example, and various control modes are actually required. However, fine control of this type requires a relatively large blower or cylinder pump in order to realize change of pressure and a flow rate freely. This causes a problem in which it is more difficult to reduce the size of the pump unit.